Questions remain about the evolutionary origins and functions of stomata. They are absent from liverworts, present to a limited extent in mosses, and are found on 410 million year-old fossils of Cooksonia, a leafless plant. Chater et al. show that orthologs of two key transcription factors that control guard cell development in Arabidopsis are expressed and required for guard cell development in the moss Physcomitrellis patens. The genes are expressed in the sporophyte but not the photosynthetic gametophyte, raising questions about stomatal function. Knocking out either gene leads to the delayed dehiscence of spore capsules, suggesting that reproduction was stomatas’ ancestral function. Nature Plants 10.1038/nplants.2016.179